With the development of the photonic network and optical switching technology, an optical device, for example, an optical switch using an optical waveguide, an optical waveguide device such as an optical modulator and so on is often used.
For example, an optical switch includes a first type using a directional coupler or a Mach-Zender interference waveguide and a second type using a branch/cross waveguide.
The first type of the optical switch operates analogically, for example, as the voltage to be applied for its switching operation increases, the intensity of the output light from the switch gradually reduces to around zero, thereafter as the voltage increases furthermore, the intensity increases again. On the other hand, the second type of the optical switch operates digitally in general, and performs an on-off operation by applying a voltage that is not less than a certain value.
The first type of the optical switch performs an on-off operation by changing the phase of light waves propagating in a waveguide. Therefore, for example, as the first type of the optical switch can perform the switching operation only by slightly changing the refractive index of a waveguide with an electro-optical effect, the first type of the optical switch has an advantage to make the operating voltage of the first type of the optical switch small and have a comparatively high operation efficiency.
However, the first type of the optical switch has a disadvantage in that it has comparatively large size. Also, in order to perform the on-off operation, it is necessary to set the operating voltage of the first type of the optical switch at an appropriate value according to the switching characteristic of the first type of the optical switch. As a result, an electronic circuit for controlling the optical switch has a complex construction, particularly, in case of a device having a plurality of switch elements such as a matrix switch, it is necessary to perform a fine adjustment of the voltage to each switch.
On the other hand, the second type of the optical switch has an advantage in that it has a comparatively small size. As it is enough to apply a voltage that is not less than a certain value in order to perform the switching operation, the second type of the optical switch has an advantage to omit an electric circuit for control and to control easily compared with the first type of the optical switch.
FIG. 1A is a top view of such a first type of the optical device, FIG. 1B is a diagram showing a section I-I of FIG. 1A, and FIG. 1C is a diagram showing a part surrounded by a dashed line in FIG. 1A. This optical device includes a core layer 1 with a cross waveguide, a clad layer 2, an electrode 3 disposed on the cross waveguide and a substrate 4, with the clad layer 2 being deposited on the substrate 4.
This optical device utilizes the total reflection phenomenon of light in the switching operation. That is, when a certain voltage is applied to or a certain current is supplied to the electrode 3, the refractive index of a waveguide just under the electrode 3 is changed by an electro-optical (EO) effect or a thermo-optical (TO) effect of a material composing the waveguide.
Due to such an operation, an input light P0 to the waveguide goes straight in the waveguide to become an output light P1, or is totally reflected by the surface just under the electrode 3 to become an output light P2.
In the case of such a second type of the optical device, however, in order to generate the variation in refractive index necessary to cause the total reflection, it is necessary to make the operating voltage of the second type of the optical switch larger than the first type of the optical device.
In this case, as it is possible to make the amount of variation in refractive index necessary to occur the total reflection small by making the branch angle θ of the cross waveguide, the efficiency of the switching operation can be improved. However, as the smaller the branch angle θ, the larger the size of the optical device, and thus there is a disadvantage to increase the loss and crosstalk remarkably.
As a measure to counter this, as described in Electronics Letters, Vol. 25, No. 11, pp.730–731, 1989, a method of suppressing crosstalk by making the core layer of a branch/cross waveguide wider in width than the core layers of an optical input waveguide and an optical output waveguide is used.
In this case, as the branch/cross waveguide has a comparatively large sectional area, the optical confinement effect in the waveguide is remarkably strong, and thus there is formed a so-called multi-mode waveguide performing the propagation in a plurality of modes.
For the purpose of this, because of a slight variation in shape and material of the waveguide of a product, a basic-mode light input from the optical input waveguide is converted into a high-order mode light resulting from the propagation through this branch/cross waveguide.
The radiation loss or coupling loss to the optical output waveguide caused by such a conversion increases, and thus the optical device may have a remarkably large loss.
Moreover, in case of changing the refractive index of the waveguide with the thermo-optical effect, the amount of variation in refractive index is in proportion to the distribution of the temperature so that the electrode 3 has the highest temperature. Therefore, as the total reflection surface of light formed by changing the refractive index of the waveguide is the same in shape as this distribution of the temperature, and is not perpendicular but inclined to the surface of the clad layer 2, a part of the light reflected by the total reflection surface is emitted from the waveguide, and thus the loss increases. Also, such a part of light may couple to the optical output waveguide opposite to the optical input waveguide, and the crosstalk may degrade.
The object of the present invention is to provide an optical device capable of reducing the optical loss and the crosstalk as well as improving the operating efficiency.